I just started syncing LR Classic (7.2 on Mac 10.13); I turned sync on, and added sync for one collection. It's working well, but at my cloud account at lightroom.adobe.com, I'm getting a bunch of iPhone photos from a few years ago. In the Photos app, and on iCloud, I don't see those photos; also don't see them in the LR mobile app on my Android (no longer have iPhone).

How do I find where these photos are syncing from? I can find them in my LR desktop app, but syncing is not turned on there.

If I should just delete these photos in the cloud, is there a way to select multiple images?

You can permanently delete from All Photos. Select the individual thumbnails and on the blue menu click delete. From a mobile device click the three dots from All Photos and choose select; then add a check-mark to each thumbnail to be deleted.